A known type of gas turbine engine, particularly for use in aircraft propulsion, is a propeller gas turbine engine or turboprop. This works in conventional form, whereby a core engine comprising compressors, combustion equipment and turbines drives one or more propeller rotor stages via a shaft from a free power, or low-pressure, turbine. The one or more propeller rotor stages may be situated at the front or rear of the engine, where front and rear are defined in terms of the direction of airflow through the engine. The propeller rotor blades extend radially outwardly to describe a larger diameter than the core engine. Each blade is pivotable about its own longitudinal axis to change its pitch and thus its angle of attack relative to the airflow. This variable pitch enables more efficient operation at a variety of operating conditions since the incident angle between the airflow and the blade surface can be optimised for the given airspeed and operating mode of the engine and aircraft.
However, one problem with providing propeller rotor stages with variable pitch blades is that the pitch may be commanded to pivot too far, or may fail with the same effect. Pitch angle is defined as shown in FIG. 1 wherein a blade 8 is shown in plan view. The blade 8 is one of a set of rotor blades rotating clockwise as viewed from the left. Thus blade 8 is travelling down the page. Pitch angle φ is measured clockwise from top dead centre. The smaller the pitch angle φ, the finer the pitch; a larger pitch angle φ means a coarser pitch. When commanded or failing too fine of the desired pitch angle the blades present a larger surface area to the airflow and restrict the flow passages between adjacent blades. In this situation the blades are driven by the airflow and transmit torque to the core engine, rather than being driven by the core engine, which causes the engine to start overspeeding. If not rapidly controlled, overspeed can cause excessive forces and result in self-destruction of the rotor stage leading to expulsion of high-energy debris. Too fine a blade angle also results in excessive drag, which has a detrimental effect on the performance of the engine and aircraft and may, at extreme angles, cause hazardous or catastrophic loss of control of the aircraft.
Conversely, if the pitch of the blades is commanded to or fails at too coarse an angle the blades begin to feather. At the extreme the blades are edge-on to the airflow and present little or no drag. However, they also exhibit a large resistance to rotation.
During operation of the engine during normal flight modes it is generally possible to prevent propeller rotor blades being driven either too fine or too coarse. However, when transitioning the engine from forward to reverse thrust, often used to provide additional braking force during aircraft landing, the blades must be rotated through fine pitch to an equivalent pitch angle in the opposite direction, e.g. −φ. Conventionally there is an overspeed limiter that acts to prevent the engine transitioning until engine torque is at a predetermined level. It is conventional to drive the rotor blades quickly through fine to the new, reverse thrust angle so that the rotor speed does not increase sufficiently to activate the limiter.
One problem with this approach is that the engine speed is usually minimised prior to transitioning to reverse thrust so that there is a greater speed margin available for the speed surge through fine pitch. This means that if the aircraft needs to go-around, and therefore demands a rapid increase in power from the engine, there is a much larger speed increase required to meet the demand. A further problem is that if there is a minor problem during transition, for example slower than normal transition, an overspeed may occur anyway. A further problem is the dichotomy between providing a larger actuation system to enable rapid engine transition from forward to reverse thrust, and the requirement for a much slower transition from reverse to forward thrust.
There are benefits to providing two stages of propeller rotor blades that rotate in opposite directions and are connected by a differential gearbox. This contra-rotation ensures that airflow leaving the stages is substantially parallel to that entering the stages. However, this may mean that if the forward propeller rotor blades are commanded or fail towards fine pitch little or no airflow can reach the rear propeller stage and little torque would be transmitted to the rear propeller stage. Similarly, if the forward propeller rotor blades are commanded or fail towards coarse pitch, there is excess torque transmitted through the differential gearbox to the rear propeller stage.